1. Field of the Invention
This invention relates generally to difference amplifiers, and more particularly, to methods for improving the common mode rejection characteristic of such amplifiers.
2. Description of the Related Art
Difference amplifiers are typically employed to measure the magnitude of a differential voltage. One common application is that of a current sensing circuit. A shunt resistor having a resistance Rshunt is connected to conduct a current of interest I, and the differential voltage Vshunt that develops across the resistor is measured with a difference amplifier, with I given by Vshunt/R.
A conventional circuit arrangement for sensing a current is shown in FIG. 1. Rshunt is connected between nodes IP and IN, and conducts current of interest I. A resistor having a resistance R1 is connected between node IP and the inverting input of an operational amplifier A1, a resistor having a resistance R2 is connected between the inverting input and the amplifier's output (Vout), a resistor having a resistance R3 is connected between node IN and the amplifier's non-inverting input, and a resistor having a resistance R4 is connected between the non-inverting input and the amplifier's output. When so arranged, Vout varies with the difference between the voltage at IP (VIP) and the voltage at IN (VIN).
Ideally, Vout varies only with difference voltage VIP−VIN. However, inaccuracies can arise in Vout due to errors resulting from finite rejection of the common-mode voltage
      V          c      ⁢                          ⁢      m        ⁡      (          =                                    V            IP                    +                      V            IN                          2              )  present at nodes IP and IN. In the described current measurement application, Vcm can be many times larger than the input voltage, necessitating a large common mode rejection ratio (CMRR). The CMRR for the circuit of FIG. 1 is given by Adm/Acm, where Adm is the circuit's differential gain, given by R2/R1, and Acm is the circuit's common-mode gain, which is ideally zero.
However, practically, Acm is never zero, and thus degrades common-mode rejection. As defined, for example, in Pallás-Areny et al., “Common Mode Rejection Ratio in Differential Amplifiers”, IEEE Transactions on Instrumentation and Measurement”, vol. 40, no. 4, August 1991, Acm is given by:
      A          c      ⁢                          ⁢      m        =            [                                                  R              ⁢                                                          ⁢              1                                                      R                ⁢                                                                  ⁢                1                            +                              R                ⁢                                                                  ⁢                2                                              +                                    R              ⁢                                                          ⁢              4                                                      R                ⁢                                                                  ⁢                3                            +                              R                ⁢                                                                  ⁢                4                                                                                        R              ⁢                                                          ⁢              4                                                      R                ⁢                                                                  ⁢                3                            +                              R                ⁢                                                                  ⁢                4                                              -                                    R              ⁢                                                          ⁢              4                                                      R                ⁢                                                                  ⁢                2                            +                              R                ⁢                                                                  ⁢                1                                                        ]        *                  R        ⁢                                  ⁢        2                    R        ⁢                                  ⁢        1              *                  1        2            .      Thus, Acm is dominated by resistor mismatch, while the contributions due to the operational amplifier itself can be made negligible by careful design.
Many methods are employed to reduce Acm. Typically, some sort of trimming is performed under a fixed set of conditions; for example, the resistors can be laser trimmed such that common-mode error is reduced. However, such trimming is costly, and is generally less effective when circuit conditions differ from those used during the trimming process.